perlPackages.Socket6: fix sv_undef compilation error

Socket6.xs redefines `PV_sv_undef`, which breaks compilation from
306d5cdf03 ("perlPackages: Add cross-compilation support."). I added a patch
to remove this redefinition. Compilation is now working again.

pkgs/development/perl-modules/Socket6-sv_undef.patch

@@ -0,0 +1,18 @@
+diff --git a/Socket6.xs b/Socket6.xs
+index 05c791c..058e9d9 100644
+--- a/Socket6.xs
++++ b/Socket6.xs
+@@ -105,10 +105,6 @@ const struct in6_addr in6addr_loopback = IN6ADDR_LOOPBACK_INIT;
+ #define	HAVE_INET_PTON		1
+ #endif
+ 
+-#ifndef HAVE_PL_SV_UNDEF
+-#define	PL_sv_undef		sv_undef
+-#endif
+-
+ static int
+ not_here(char *s)
+ {
+-- 
+2.16.3
+

pkgs/top-level/perl-packages.nix

@@ -13549,6 +13549,7 @@ let self = _self // overrides; _self = with self; {
     };
     setOutputFlags = false;
     buildInputs = [ pkgs.which ];
+    patches = [ ../development/perl-modules/Socket6-sv_undef.patch ];
     meta = {
       description = "IPv6 related part of the C socket.h defines and structure manipulators";
       license = stdenv.lib.licenses.bsd3;